Coagulase Negative Staphylococci (CoNS): A Review

Coagulase-negative staphylococcus (CoNS) has gained more importance as pathogenic organism for infections in both human and animals. CoNS are especially prevalent in immunocompromised patients, critically ill patients, patients having invasive medical devices. The incidence of CoNS varied across different geographic locations in humans and animals. Also, there are varying antibiotic resistance patterns observed in CoNS species, with high methicillin resistance and cross resistance against many antibiotics. Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus xylosus are the most commonly reported species in various studies. Various virulence factors in CoNS are responsible for enhanced pathogenicity. Because of advancement in diagnostic techniques understanding of molecular mechanisms of CoNS pathogenicity is possible. Recent advances in identification and typing methods, virulence screening methods will help to assess true pathogenic potential of CoNS species. This review focuses on various CoNS species, their identification and virulence factors and clinical importance.


Coagulase-negative
Staphylococci (CoNS) classified as mere contaminants, are becoming clinically relevant because of widespread of antibiotic resistance, biofilm formation and increased use of medical devices such as Central venous line, urinary catheter, Prosthetic valves etc. As there is marked species diversity in CoNS, there is need for increased laboratory capacity for effective speciation.
Coagulase-negative Staphylococci (CoNS) are normal flora of human skin and mucous membranes, they have previously been Considered nonpathogenic or contaminant having little clinical significance [1]. But now they have been Considered as significant potential pathogen responsible for hospital acquired infection because of widespread antibiotic resistance and increasing use of medical devices and occurs specially in immunocompromised patients and patients having indwelling devices.
Because of bioflim formation on medical devices, majority of hospital acquired infections are caused by CoNS. Biofilm formation also increases the resistance to antimicrobial agents and host defense mechanisms and because of that, it is very difficult to eradicate biofilm associated infections by conventional antibiotic treatment [2][3][4].

Habitat
CoNS is a normal flora of skin and mucous membranes of humans and animals [10,11]. Table 3 shows colonizing areas of different CoNS species.

Transmission
Maximum CoNS infections are hospital-acquired or health-care related infections as they have the ability to survive in ICU(Intensive care unit), on medical devices and equipments for months [16,17,18]. Some clones are probably endemic in the hospital environment [18,19]. The mecA gene carriage in these clusters is usually very high, which suggests that antibiotic resistance is one of the major selective forces [20][21][22][23].
Emergence and spread of CoNS in hospitals is dependent on duration of hospital stay (especially ICU stay), Antibiotic treatment period,antibiotic pressure in the environment and hygiene standards [16]. Hand hygiene precautions is extremely important for preventing nosocomial colonization and infections.

Risk Factors for CoNS Infections
Risk factors for CoNS infections includes medical conditions such as [24] immune suppression, premature birth, neutropenia, dependence of renal dialysis, malignancy, cardiothoracic surgery and long term hospitalization.

Morphology
CoNS are gram-positive, nonmotile, non-sporeforming cocci. They are usually arranged in irregular (grape-like) clusters or singly, in short chains (three or four cells), in pairs or tetrads.

Classical approach for separation of CoNS from coagulase positive Staphylococci
Coagulase can contribute to pathogenicity by inhibiting the bactericidal activity of normal serum and by inhibiting phagocytosis through deposition of fibrin on the bacterial cell walls. In the laboratory, two types of coagulase tests are used such as slide test and tube test. Table 2 shows all the coagulase positive and coagulase negative Staphylococci species.

Grouping of CoNS by novobiocin testing
For CoNS isolates which have been recovered from urinary tract specimens, novobiocin resistance is used to distinguish the intrinsically resistant S. saprophyticus subsp. saprophyticus from other clinically important CoNS, using a 5 ug novobiocin disc on Mueller-Hinton agar [25].

CoNS species and subspecies
At present, there are 32 recognized species and eight subspecies present in the genus Staphylococcus ( Table 2) and about one-half of these are indigenous to humans.

Virulence Factor in CoNS
CoNS are seldom life-threatening except in immunocompromised patients as CoNS do not produce aggressive virulence factors [1].

Capsule
Among CoNS, capsule formation is frequent and they possess increased virulence compared to non-encapsulated variant strains. Slime may contain capsular polysaccharides, proteins and cell wall components. The capsule confers resistance to phagocytosis [26].

Slime
Glycocalyx is Considered a slime layer when glycoprotein molecules are loosely attached with the cell wall. Slime material and biofilm formation has important role in colonization of uroepithelium and medical device-associated infections [27]. Slime has also been shown to inhibit the cell mediated immune response in vitro.

Biofilm
Biofilm structures comprises mainly bacterial cells and an extracellular polymeric substance (EPS) provided by the polysaccharide intercellular adhesion (PIA) .PIA synthesis is associated with intercellular adhesion operon (ica ADBC) [28].
Biofilm provides protective environment to microorganisms and responsible for quorum sensing( the exchange of genetic material between cells and intercellular communication) [29]. Micro-organisms becomes more resistant to antibiotics and to host defense mechanisms due to biofilm.

Cytolytic toxins
Delta-toxin (PSM is produced by S. epidermidis. It forms pores in the cell membrane which leads to erythrocytes and other mammalian cells lysis [25].

Production of Lantibiotics
Antibiotic-like peptides produced by commensal staphylococci are called lantibiotics and belongs to the class of cationic antimicrobial peptides (CAMPs) and are active against gram-positive bacteria. Lantibiotics production has role in bacterial interference on skin and mucous membranes. Type A lantibiotics induce pores in the cytoplasmic membrane. Lantibiotics produced by S.epidermidis are epidermin, Pep5, epilancin K7, epidermicin NI01, and epicidin 280. Other species such as S. gallinarum (gallidermin), S. hominis (hominicin), and S. warneri (nukacin ISK-1) also show lantibiotic production [25].

Siderophore
Microorganisms produce low molecular weight (<1000D) chelating compounds called siderophore in their iron especially in free form. Siderophores are helpful to overcome host's nonspecific defense mechanisms and thus helpful in survival within the host [30].
Meiwes et al [31] has detected two iron binding compounds, staphyloferrin A and B which were highly hydrophilic and anionic.

Extracellular enzymes
CoNS produces variety of enzymes and extracellular proteins such as proteases, lipases, phospholipases, esterase's, protein A, and fatty acid modifying enzymes. Protease are responsible for proteolytic inactivation of antibodies, platelet microbicidal proteins, and destruction of tissue protein which leads to increased invasiveness. S. epidermidis has two lipase genes involved in skin colonization [32].

Exopolymers
Polysaccharide intercellular adhesin (PIA) and poly gamma-glutamate (PGA)s are produced by S. epidermidis.  1958 Smith and coworkers First reported pathogenicity of CoNS in patients with septicemia [3]. 1965 Wilson and Stuart Identified CoNS in pure culture form [4]. 1962 Pereira UTIs were caused by certain group of CoNS which is now known as S. saprophyticus [5]. 1971 Pulverer and Pillich(Cologne, Germany) Investigated pyogenic infections in Cologne, Germany and reported 10% infections were due to CoNS and CoNS were found in pure culture [6]. 1971 Holt Reported that CoNS were responsible for colonization of ventriculoatrial shunts followed by septicemia [7]. Axilla, inguinal and perineal areas, anterior nares, conjunctiva, and toe webs [12]. S. hominis S. haemolyticus axilla and pubic region [12]. S. capitis Fore-head and scalp following puberty [13]. S. lugdunensis Pelvic and perineum regions, lower extremities, axillae [14]. S. saprophyticus subsp. saprophyticus Rectum and genitourinary tract [12] S. auricularis Human external ear [15].    PIA has similar functions as PGA and also protects against complement deposition and immunoglobulins [33].  Table  6 shows various biochemical characteristics of CoNS. As there is addition of more discriminating tests and availability of growing data bases, the reliability of these commercial systems will continue to increase [34].

CONCLUSION
CoNS is already causing a significant level of infection and morbidity. It won't take long before it starts having huge impact on the immunocompromised patients, with the increasing use of foreign materials like prosthetic valves, catheters, central lines and other medical advances. Additional factors like increasing antimicrobial resistance and virulence in the species might limit its treatment. Thus it's necessary to study CoNS at species level to understand their role as reservoir of virulence and resistance genes. Also it will help develop colonization preventing materials for various uses.

CONSENT
It is not applicable.

ETHICAL APPROVAL
It is not applicable.